Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions		Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		$ 11,079.7	$ 3,333.3
Gross Unrealized Loss, Less than 12 Months		627.7	69.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		11,461.8	15,553.4
Gross Unrealized Loss, Greater than 12 Months		2,823.9	2,436.8
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		268.4	21.8
Gross Unrealized Loss, Less than 12 Months		17.7	0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		216.3	235.6
Gross Unrealized Loss, Greater than 12 Months		31.6	15.6
States, Municipalities, and Political Subdivisions
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		506.8	138.2
Gross Unrealized Loss, Less than 12 Months		26.2	3.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,487.0	1,922.2
Gross Unrealized Loss, Greater than 12 Months		542.4	462.8
Foreign Governments
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		304.3	264.6
Gross Unrealized Loss, Less than 12 Months		12.0	5.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		315.4	318.2
Gross Unrealized Loss, Greater than 12 Months		178.6	151.8
Public Utilities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		1,725.0	642.1
Gross Unrealized Loss, Less than 12 Months		101.0	19.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		1,337.8	1,710.3
Gross Unrealized Loss, Greater than 12 Months		321.4	270.7
Mortgage/Asset-backed Securities
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	[1]	798.3	206.6
Gross Unrealized Loss, Less than 12 Months	[1]	21.8	0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	[1]	100.1	275.3
Gross Unrealized Loss, Greater than 12 Months	[1]	15.7	16.9
All Other Corporate Bonds
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		7,473.0	2,060.0
Gross Unrealized Loss, Less than 12 Months		448.9	40.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		8,001.4	11,087.9
Gross Unrealized Loss, Greater than 12 Months		1,734.0	1,518.9
Redeemable Preferred Stocks
Debt Securities, Available-for-sale, Unrealized Loss Position
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months		3.9	0.0
Gross Unrealized Loss, Less than 12 Months		0.1	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		3.8	3.9
Gross Unrealized Loss, Greater than 12 Months		$ 0.2	$ 0.1

[1] Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types